Interaction Of Coal Fracture And Matrix On Coal Permeability

The coal bed gas resources in China are complicated in terms of occurrence conditions,and the proportion of high stress,structural coal and low permeability coalbed methane is high.Although a variety of fracturing techniques can be used to improve the permeability of coal seam in the short term,the gas production is attenuated greatly in the later period of concentration and restricts the large-scale industrial exploitation of coal bed gas.The process of coalbed methane exploitation involves multi-scale migration of pore fluid.At micro scale,gas adsorption/desorption diffusion occurs in the micropore of coal matrix,while at macro and mesoscale scale,coal fracture gas is dominated by Darcy flow.For the prediction method of coalbed methane production,most of them first calculate the flow rate according to the coal permeability,and then determine the total gas production.Therefore,studying the variation law of coal permeability during gas migration is very important for predicting coalbed methane production and coal mine gas management.The permeability of coal represents the conducting fluid capacity of coal.Because coal has dual pore structure,coal permeability is usually classified as coal fracture permeability.Although the study not only considers the effect of external stress and overall strain on permeability,it also considers the deformation of coal matrix caused by adsorption expansion/desorption,but neglects the influence of coal fracture deformation and matrix deformation on permeability under the influence of effective stress.Compared with the seepage process in the crack,the gas diffusion process in the matrix takes a long time to reach the equilibrium state,which will lead to the lag of pore pressure on the change of pore pressure.The synchronous variation of effective stress between matrix and crack will make matrix deformation affect fracture opening and further change coal permeability.Therefore,in this paper,the effect of coal matrix gas diffusion on permeability is studied,and the experimental study of coal permeability is carried out by using the self developed CBM seepage test system,and the response law of pore pressure change to the coal fracture matrix system is discussed.The main research work and achievements are as follows:(1)Pore pressure is applied to the experiment of coal deformation,and the law of the influence of gas seepage diffusion process on coal deformation is studied.Through the injection of helium into coal samples,it is observed that the deformation process of coal is closely related to the gas migration process.Whether the coal is in the fixed displacement boundary constraint or the constant stress boundary constraint,in the gas injection process,the gas seepage can lead to the increase of the coal crack opening,and the gas diffusion leads to the expansion of the coal matrix.Under the constraint of fixed displacement,the deformation of coal mass first expands and then shrinks.Under constant stress constraint,the coal deformation is manifested as two stages of expansion.With the increase of gas injection pressure,the conversion time from seepage expansion to diffusion expansion is shortened,and the diffusion time of matrix is prolonged.(2)To carry out the experiment of coal permeability evolution with time,and study the influence rule of effective stress and slippage effect on coal permeability.First,a coal fracture matrix interaction model is established,and the mechanical parameters of coal matrix and coal fracture are obtained.Secondly,based on the transient test method,the coal permeability increases with the increase of gas injection pressure,while the coal permeability increases with time and then decreases with time.The results confirm the influence of dual effective stress on coal permeability.In addition,the steady state test method is used to observe that coal permeability is controlled by effective stress and slippage effect with decreasing pore pressure.(3)To carry out the experiments of coal deformation and permeability in the process of gas injection under constant volume,and to study the response of coal matrix fracture interaction to permeability evolution under the fixed boundary conditions.It is observed that after injection of helium,the local strain of coal increases rapidly and then shrinks,and the ratio of shrinkage strain to expansion strain is 0.5.According to the geometric model of coal fracture matrix system(match rod model),the permeability model controlled by local strain under constant volume constraint condition is derived.By comparing the experimental data of coal permeability and the prediction value of the model,it is found that the Ma-Harpalani model will overestimate the permeability because of neglecting the difference between the fracture pressure and the matrix pressure.The local strain model can predict permeability better.In addition,it is confirmed that in the process of unsteady gas flow,the Biot coefficient is less than 1,which can accurately describe the influence of coal fracture medium system pressure on permeability.